1. Field of the Invention
The present invention relates to an ink jet recording head for discharging ink to a recording medium through an ink discharge port, a method of manufacturing the ink jet recording head, and a recording apparatus having the ink jet recording head.
2. Related Background Art
U.S. Pat. Ser. Nos. 4,723,129 and 4,740,796 have disclosed ink jet recording methods (that is, bubble jet recording methods) of a type which is capable of recording a high quality and precise image at high speed and high density and which can be easily adapted to a color recording system and to a compact size apparatus. Therefore, they have attracted attention in recent years. A typical example of an apparatus which employs the aforesaid method has a heat acting portion which cause heat to act on ink in order to discharge recording liquid (hereinafter called "ink") by utilizing heat energy.
That is, an electrothermal transducer element having a pair of electrodes and heat generating resistors, connected to the electrodes and arranged to generate heat to be supplied to the region interposed between the electrodes, is disposed to correspond to an ink passage so as to utilize heat energy generated by the heat generating resistors so as to rapidly heat the ink present on the heat acting surface to form bubbles which cause the ink to be discharged.
Since the heat acting surface of the ink jet recording head is subjected to severe conditions such as mechanical shock caused from cavitation that takes place due to repetition of generation and disappearance of the bubbles of the ink, and also subjected to erosion, and the temperature is rapidly raised and lowered in a range of about 1000.degree. C. in an extremely short time of 0.1 to 10 microseconds, a layer for protecting the heat generating resistor from the environment, in which the ink jet recording head is used, is necessary. The protection layer must have excellent heat resisting, liquid resisting, liquid permeability preventing, acid stability, insulating, damage resisting and heat conductive characteristics and therefore an inorganic compound such as SiO.sub.2 is ordinarily employed. Since a protection layer constituted by a single layer is sometimes unsatisfactory to protect the heat generating resistor, a protection layer for protecting the outermost layer which serves as the heat acting surface is sometimes made of metal such as Ta.
The protection performance of the protection layer is a critical factor for the life of the ink jet recording head. Furthermore, the heat generated by the heat generating resistor is efficiently transmitted to the heat acting surface by a so-called heat resisting layer formed below the heat generating resistor and having a low heat conductivity.
The conventional ink jet recording head structured as described above is formed in such a manner that the heat resisting layer, the electrothermal converter and the protection layer are stacked on either side of a flat and smooth supporting substrate, that is, on the surface adjacent to the heat acting surface. The aforesaid layers and the electrothermal converter are formed by any one of a vacuum evaporation method, a sputtering method, a CVD method, a spray method, a thick-film coating method, and the like. In order to obtain desired performance, a proper method is selected from the aforesaid methods.
On the other hand, the supporting substrate has a function for mechanically holding the heat resisting layer, the heat acting portion and fluid passages formed thereon and a function for quickly dispersing excessive heat conducted thereto. Therefore, the supporting substrate is typically made of single crystal Si or alumina glades. The aforesaid heat resisting layer is ordinarily formed by oxidizing the surface of the supporting substrate with heat.
However, it is very difficult for a conventional ink jet recording head of the type structured as described above to prevent a defect taken place at the time of forming the thin films because it is formed by stacking a plurality of the thin films on the supporting substrate. In particular, a defect of the protection layer critically deteriorates the life of the recording head. If the protection layer is thickened in order to prevent the defect, the heat conductivity deteriorates, causing the electric power consumption to be enlarged and heat to be stored undesirably. Hence, bubbles cannot be formed stably. What is worse, the enlargement of the electric power consumption will cause the temperature of the recording head to be changed excessively at the time of the operation of the recording apparatus. The temperature change will undesirably change the density of the recorded image by a degree corresponding to the change of the temperature. In addition, the unstable bubble formating causes the volume of the ink droplet to be changed, and also causes the density of the recorded image to be changed.
The aforesaid problem, that is, a change in the density of the obtained image, does not meet the requirement to improve the quality of the recorded image, and therefore the problem must be overcome as soon as possible. The heat generated by the heat generating resistor is transmitted toward the heat acting surface, that is, in the vertical direction and in the horizontal direction. The horizontal directional transmission of the heat raises the temperature of the recording head, causing the density of the recorded image to be undesirably changed. What is worse, the aforesaid method of forming the heat acting portion by stacking a plurality of the layers inevitably generates a stepped portion in the protection film. Since the quality of the film of the stepped portion is inferior to that of the flat portion, another problem arises in that a defect will be generated in the stepped portion.
In the conventional structure in which the electrode layer is formed on the heat generating resistor, the electrode layer is desired to be thickened in order to prevent the loss of the electricity taken place in the electrode. However, a limitation is present in thickening the electrode layer because forming of the aforesaid step in the protection layer must be prevented. Furthermore, the electrode pattern cannot be widened sufficiently because there has recently been an increased desire to record an image at high density. As described above, the high density recording encounters the noted design limitation and a limitation in terms of ease of manufacturing.
That is, the number and the size of foreign matter particles (points where the defects originate) generated at the time of the pattern forming process and the handling process during the process of forming the films are relatively enlarged in inverse proportion to the size of the formed pattern although each of the number of the foreign matter particles and the size has a predetermined distribution. Therefore, the subject facility must be further cleaned up in order to remove the foreign matters. Furthermore, the cleanliness must be maintained also during the manufacturing process. If a satisfactory cleanliness cannot be realized, the manufacturing yield will deteriorate. Therefore, the cost to maintain the cleanliness will raise the overall cost of the recording head.